Pre-pro-B cell growth stimulating factor (PPBSF) is a novel hybrid cytokine consisting of IL-7 and the beta-chain of hepatocyte growth factor/scatter factor (HGF/SF). Our working hypothesis is that PPBSF selectively stimulates self-replication, proliferation and differentiation of pre-pro-B cells by effectively transducing signals in the presence of low levels of IL-7R; and that it "primes" pro-B cells to respond to monomeric lL-7 by upregulating the expression of IL-7Ralpha-chain. Our hypothesis further proposes that PPBSF, through its IL-7 component, renders pleiotropic HGFbeta lineage-specific, thereby directing the commitment of hemopoietic stem cells to the lymphoid pathway. The Specific Aims are to: 1) trace the synthesis, assembly and display of PPBSF in/on BM stromal cells; 2) define the functions of rPPBSF on B-lineage cells in vitro; 3) characterize the receptor(s) for PPBSF; 4) confirm the existence and functions of PPBSF in vivo; and 5) determine if PPBSF can enhance B-lineage engraftment. Large scale production of rHGFbeta will be accomplished with transfected mammalian cell lines, and purified rPPBSF will be generated by self-assembly of the rIL-7 with rHGFbeta heterodimer. Full-length cDNA will be isolated to determine if HGFbeta is formed by alternative splicing of the HGF gene and/or independently of the HGFalpha-chain; and the protein products will be analyzed in immunopreciptiation experiments and by in vitro translation to determine if PPBSF is assembled intracellularly or extracellularly. The receptor(s) for PPBSF will he characterized after selective precipitation o ligand-receptor complexes. The ability of rPPBSF to induce proliferation, phenotypic differentiation, D-J and V-D-J rearrangement, upregulation of IL-7Ralpha, TdT and c mu and formation of high affinity IL-7R on purified HSCs, pre-pro-B cells and/or pro-B cells will be evaluated in vitro. The presence and functions of PPBSF in vivo will be demonstrated by in situ and ex vivo irnmunostaining; mRNA analysis of BM extracts; and infusion of rPPBSF or antibodies thereto. These studies may have both basic and clinical implications concerning early B cells development, BM transplantation, acute lymphoblastic leukemia, and selective immunodeficiency disorders.